Spirochetes are a group of medically important bacteria which have been poorly studied. We do not understand some of the most fundamental aspects of nutrition, structure and function, and mechanisms of pathogenesis. One unique aspect of all spirochetes is their rapid motility and distinct structure. As compared to other flagellated bacteria, where the flagella are external to the cell wall, spirochetes contain flagella-like structures (axial filaments) which are internal. That is, these filaments and the bacteria are ensheathed by a membrane. It is difficult to understand at this time how they propel themselves. We do not even know if these axial filaments play a role in motility. We propose to use a genetic-structural and biochemical approach towards understanding how spirochetes move. We are using Leptospira as our model system. Our approach is to mutagenize Leptospira and isolate colonies which do not spread on agar. We have found that many motility mutants isolated synthesize aberrant axial filaments as observed by electron-microscopy. This is the first suggestive evidence that axial filaments play a role in spirochete motility. We are now characterizing these mutants. The results have permitted us, in collaboration with Howard Berg of the University of Colorado, to propose a model of Leptospira motility.